This could be the first instance of a U.S.-based internet company obeying such a request, which is believed to have come from the National Security Agency or FBI.

Two former Yahoo employees, as well as a third person familiar with the company’s actions, told Reuters, “The company complied with a classified U.S. government directive, scanning hundreds of millions of Yahoo Mail accounts.”

It’s not clear what information Yahoo might have been helping federal agencies look for, but Reuters did discover the company was searching for a very specific set of characters. It’s also not clear whether Yahoo actually handed over any data, just that the company was using custom built software to look.